It is common practice to protect electrical power distribution systems against short circuits or other causes of overloading. Circuit breakers are used for this purpose in conjunction with suitable means for instantaneously sensing an overload. The circuit breakers typically include relatively heavy and slow moving mechanical parts. Accordingly, available circuit breakers have a time delay corresponding to the duration of a few cycles of a 50 Hertz AC supply. Even within such a short period of time, the overload can cause damage to electrical equipment at the end of the power transmission line. Moreover, there is a risk of the circuit breaker itself being damaged by a short circuit current such as it fails without breaking the circuit. Extensive damage can then be done to equipment at the end of the power transmission line. A generally Known way of rapidly reducing a short circuit current is to connect a current limiting element in series with a circuit breaker.
Such devices, called "fault current limiters", are Known to make possible simplification of the construction of the circuit breaker. Different kinds of fault current limiters are Known, including ones based on the use of a superconductor and a parallel current limiting resistor or reactance.
An article in Journal of Applied Physics, 49(4), April 1978, pages 2546 to 2550 by K E Gray and D Fowler entitled "A Superconducting Fault Current Limiter" describes such a device. The operation of the device depends on the creation of a magnetic field which partially changes with the magnetic flux density around the conductor. The device is discussed in detail in EP-A-O 345 767 which comments on its complexity and on the fact that it employs three separate parameters, namely temperature, current density and magnetic flux density to achieve the desired current limitation. EP-A-O 345 767 itself relates to a superconducting fault current limiter in which only the critical current density is used to cause a superconductive material to pass from the superconducting to the non-superconducting state.